1. Field of the Invention
This invention relates to improvements in current limiting circuitry and methods, and more particularly to improvements in current limiting circuits and methods that may be used in RS485 type circuits, or the like.
2. Relevant Background
Many circuits are constructed with at least two channels or sections that do not conduct simultaneously. Although many classes of such circuits exist, one class of such circuits are those that are designed according to the so-called "RS485" standard. Circuits of this type may have, for example, two or more drivers that source and/or sink out-of-phase outputs.
Such drivers may include, for example, "high side driver circuitry" in which two drivers are provided, only one of which being on any instant in time to source current to another circuit or application. In some circuits, "low side drivers" may also be included, in which two driver stages are provided to sink current from an application; again however, only one stage is operated to conduct at any instant in time. In some circuits of this type, both high and low side drivers are provided. In these cases, only one of the two high side drivers is on, and only one of the two low side drivers is on at any instant. Furthermore, although in an elemental sense, only a single stage is described, it is understood that typical circuitry of the type described may have a large number of stages associated with a single integrated circuit device.
In many applications involving circuits of this type, it is desirable to provide for a current limiting function in the circuit. Typically to provide for such current limiting function, a current limiting circuit is provided for each driver in the circuit. Thus, four current limiting circuits are required in a circuit of the type described having two high side drivers and two low side drivers. When the circuit is integrated onto a monolithic integrated circuit chip, the current limiting function occupies a large amount of the chip area that would otherwise be available for integrated circuit components. Moreover, when such current limiting circuitry is provided for each individual driver, the quiescent current demands may be significant.
What is needed therefore, is method and circuit for providing current limiting functions with minimized integrated circuit chip area requirements for circuits of the type described in which a plurality of current driver circuits are provided, but which source and/or sink at least partially out-of-phase output currents.